The present invention relates to a distributed control system in which a plurality of control apparatuses are connected to a common transmission line, and especially to a filtering method and a filtering circuit used in the distributed control system, for selecting and receiving messages transmitted on the common transmission line.
In a process automation system or a factory automation system, many apparatuses each of which includes a processor, what is called xe2x80x9cfield apparatusxe2x80x9d, are used for sensors to detect physical quantities such as pressure, temperature, or flow rate, motors to drive mechanical equipment, photo-sensors to detect positions. What is called a distributed control system controls an object system such as a plant by connecting the above field apparatuses, PCs (Programmable Controllers) for controlling the filed apparatuses, computers with input/output devices and display devices, and so on to a common transmission line so that those field apparatus, PCs, computers, etc., mutually send or receive messages via the transmission line.
In a distributed control system, for example, a physical quantity data from one sensor is simultaneously received by a plurality of motors, and each motor which has received the data independently executes processing by using the data from the sensor in real time. Therefore, data used for controlling of each field apparatus (referred to as a message) need to be transmitted in a very short time. Further, an extension or a change of a plant recently tends to be frequently carried out, which causes a frequent change in the composition of the control system for a plant. Accordingly, the easiness in extending or replacing field apparatuses or PCs has been strongly desired.
To satisfy the above requirements for a distributed control system, a conventional distributed control system uses a content-code transmission method in which each message with a content-code indicating the content of data to be transmitted is broadcasted on a comon transmission line, and each node connected to the transmission line selects and receives the broadcasted message based on the content-code attached to the message. An example of the content-code using transmission method is disclosed in Japanese Patent Application Laid-Open Sho. 56-111353. Further, a distributed control system using the content-code transmission method is disclosed in Japanese Patent Application Laid-Open Hen. 1-124050. In this distributed control system, a received message includes an identifier of the kind of this message (message for on-line control or test) in addition to a code indicating the content of data to be transmitted and a node sending this message. Each node determines whether or not a received message is to be stored in the node, based on the above message-sending conditions included in the message.
According to recent development of a semiconductor technology, an intelligent function of a field apparatus has made further progress. Thus, although a conventional field apparatus simply detects or measures a physical quantity, a newer type of field apparatus performs complicated processing such as a self-diagnosis, a feed-back control, and so on. Consequently, it is required that results of a self-diagnosis performed in respective field apparatuses are collected by a computer which is a host apparatus of the field apparatuses, a plurality of parameters are set to each field apparatus, and similar net-work processing. Accordingly, in addition to the conventional data transmission in which messages of short data length such as data used to control an object system such as a plant, (referred to as control data), are transmitted on a common transmission line connecting a host computer and field apparatuses in a short transmission time, it has been required that messages with a long data length and of types different from those of the above control data, (referred to as information data), which need not be always transmitted in a short transmission time, are also transmitted along with control data on the transmission line in a recent distributed control system.
The above content-code transmission method is used to transmit control data to correspond to requirements of the easy extension and the synchronized data transmission of a distributed control system. On the other hand, since information data need not be simultaneously transmitted to a plurality of nodes, it is adequate to send information data by designating a message-receiving node, which is called a one-to-one transmission method. However, since the data length of information data is long, it takes much time for the node which has received the information data to process the information data, and the computing load of the node also increases.
In the content-code using transmission method in conventional techniques, data are broadcasted by attaching a content-code indicating the content of data to each data to be transmitted without designating a data-receiving node. Therefore, since data cannot be sent by designating the node receiving the data in the content-code transmission method, the content-code transmission method and the one-to-one transmission method cannot be used together.
Further, a distributed control system using the content-code transmission method has a problem in that since each control unit in the distributed control system receives all the message broadcasted on a transmission line, and determines whether or not each of the received messages is to be taken into the unit based on the attached content-code, the load for the transmission processing in each node is large. Especially, in a field apparatus performing component control and transmission processing with one CPU, if the load of the transmission processing to which the content-code transmission method is applied greatly increases, it affects processing of an application program to execute the component control. In the worst case, the timing or the period of the processing of an application program cannot be kept, which causes a failure in the normal control of a plant, etc. Furthermore, such a failure in the normal control frequently results in the serious deterioration in the safety of a plant or in the yield of a production line in a factory.
The first object of the present invention is to provide a method and an apparatus resolving the above problems, in which transmission methods of different types such as the content-code using transmission method, the one-to-one transmission method, etc., can be used together, and data of different types such as control data, information data, etc., can be transmitted on the same transmission line.
The second object of the present invention is to provide a method and an apparatus capable of preventing increase of the load of transmission processing from affecting other processing such as execution of an application program.
To attain the first object, the present invention provides a first distributed control system in which a plurality of nodes are connected to a transmission line, each node controlling components connected to this node and transmitting a message to other nodes,
wherein each node sends a message including at least two message-sending condition-identifying portions, one of the condition-identifying portions including data indicating a message-receiving node or non-designation of a message-receiving node; takes in each of messages transmitted on the transmission line into a comparison and selection means provided in this node; and determines whether or not the taken-in message is to be received in this node.
In the first distributed control system, the determination of whether or not the taken-in message is to be received in this node is executed based on a corresponding situation between contents of the condition-identifying portions and data of message-receiving conditions registered in the comparison and selection means.
In the first distributed control system, by further providing a declaration region for describing a position of each condition-identifying portion and the length of data set to the condition-identifying portion in a message sent by each node, the number of the condition-identifying portions can be variably set.
Further, in the first distributed control system, the comparison-selection means includes registration parts for registering data of message-receiving conditions, at least tow registration parts for comparing contents of the condition-identifying portion in the taken-in message with the data of message-receiving conditions registered in the registration parts, a received-message storing part for storing a message to be received in this node, a control part for determining whether or not the taken-in message is to be received in this node based on the result of comparison-processing performed in the comparison and selection means and for transferring the taken-in message to the received-message storing part if it is determined that the taken-in message is received in this node, otherwise, abandoning the taken-in message.
Furthermore, in the first distributed control system, the comparison and selection means includes registration parts for registering data of message-receiving conditions, at least tow comparison parts for comparing contents of the condition-identifying portions in the taken message with the data of message-receiving conditions registered in the registration parts, received-message storing parts of the same number as that of the comparison parts, for storing a message to be received in this node, a control part for determining whether the taken-in message is to be received in this node based a result of comparison and processing performed in the comparison and selection means, and for transferring the taken-in message to a corresponding one of the received-message storing parts if it is determined that the taken-in message is received in this node, otherwise, abandoning the taken-in message.
Additionally, in the first distributed control system, the comparison and selection means includes a plurality of base filtering circuits, each base filtering circuit one of comparing message-sending conditions set to a taken-in message from the transmission line with one of message-receiving conditions registered in advance and outputting the taken-in message if agreement is established in the comparison, and the at least two base filtering circuits are serially connected to each other in each set composed of at least two base filtering circuits.
Also, in the above distributed control system, the base filtering circuit includes an input-message storing part for including a message input from an external circuit, a registration part for registering message-receiving conditions, a comparison part for comparing a content of one of condition-identifying portions in the take-in message with the registered message-receiving conditions, a control part for determining whether or not the taken-in message is to be received in this node based on a result of the comparison, and an output-message storing part for storing a message to be received and for outputting the stored message to an external circuit.
On top of that, in the first distributed control system, each message includes an arbitration region composed of a condition-identifying portion indicating a data-kind and a condition-identifying portion indicating a message-sending node, and a data region composed of a condition-identifying portion indicating a message-receiving node and data to be transmitted; and a processor provided in each node includes a message storing part for storing a message taken-in from the transmission line, a first comparison part for performing comparison-processing for the arbitration region, a second comparison part for performing comparison-processing for the data region, and a control part for determining whether or not the message is to be received in this node based on results of the comparison-processing performed in the first and second comparison parts, and for transferring the taken-in message to be received in this node to the received-message storing part.
Also, in the above distributed control system, the arbitration region is arranged at top position of a message sent be each node, and the processor controls sending of a message to another node based on bit-configuration in the arbitration region of the taken-in message transmitted in a serial transmission manner.
Moreover, the present invention provides a second distributed control system, wherein a message sent by each node includes predetermined address data in each condition-identifying portion; a comparison and selection means provided in each node includes a message storing part for storing a message taken-in from the transmission line, a registration part registering a plurality of flags, a flag being attached to each address, a received-message storing part for storing a message to be stored in this node, and a control part for determining whether or not the taken-in message is to be received in this node; and the control part determines whether or not the taken-in message is to be received based on a flag attached to an address indicated by a content of each condition-identifying portion in the taken-in message, and the control part further transfers the taken-in message to the received-message storing part.
Also the present invention provides a third distributed control system wherein each node sends a message including at least two message-sending condition-identifying portions, one of the condition-identifying portions including data indicating a message-receiving node or non-designation of a message-receiving node; and a controller provided in each node includes a processor for controlling circuits in the controller and message-transmission with other nodes, a plurality of transmission processors, each transmission processor determines whether or not the taken-in message is to be received in this node based on a corresponding situation between contents of the condition-identifying portions and data of message-receiving conditions registered in this transmission processor, and a transceiver connected to the transmission line, for taking in each of messages transmitted on the transmission line and for sending the taken-in message to the transmission processors.
Next, to attain the second object, the present invention provides a method of message transmission performed by each node in a distributed control system in which a plurality of nodes are connected to a transmission line, each node controlling components in this node and transmitting a message to other nodes, the method comprising the steps of:
sending a message including at least two message-sending conditions by setting data indicating a message-receiving node or non-designation of a message-receiving node in one of the at least two condition-identifying portions,;
taking in a message from the transmission line; and
filtering the taken-in message, this step comprising the steps of:
checking for a corresponding situation between contents of the condition-identifying portions and data of message-receiving conditions registered in this node; and
determining whether or not the taken-in message is to be received in this node based on results of the checking.